Bipolar transistors are generally constructed from two pn junctions lying close together in a semiconductor crystal. In different configurations, either two n-doped regions are separated from one another by a p-doped region (npn transistors) or two p-doped regions by an n-doped region (pnp transistors). The three differently doped regions are referred to as the emitter, the base, and the collector. Therefore, a bipolar transistor is essentially a three terminal device having three doped regions of alternating conductivity type.
Bipolar transistors exhibit desirable features such as high current gain and an extremely high cut-off frequency for switching applications, and high power gain and power density for microwave amplifier applications. These features make bipolar transistors important components in logic circuits, communications systems, and microwave devices.
As with other types of semiconductor devices, there is a demand for bipolar transistors having increasingly higher operating frequencies and/or switching speeds. Since their invention in 1947, many attempts have been made to meet these demands and improve the performance of such transistors with respect to their speed, power, and frequency characteristics. These attempts have focused on making devices better suited for high speed applications such as microwave and logic devices. One particular way to meet, these demands for higher frequency operation is to provide a device with a lower base resistance and base-collector junction capacitance.
In the case of bipolar transistors, the base-collector capacitance is one of the decisive transistor parameters which determine important characteristic quantities of the bipolar transistor such as the maximum oscillation frequency. The extrinsic base resistance corresponds to the resistance between the base, or the actual base area, and an external contact, which is connected to the base via a connecting line.
Accordingly, for example, the upper limit for application frequency of a bipolar transistor (e.g., heterojunction bipolar transistor) can be approximated as:
      F    max    =                    f        T                    8        ⁢                  π          ·                      R                          B              ·                              C                BC                                                        
where fmax designates the maximum oscillation frequency, fT designates the transition frequency, RB designates the base resistance, and CBC designates the base-collector capacitance. The transition frequency, fT, is essentially determined by the dopant profile in the active transistor region while the product of RB. CBC can be influenced by the transistor layout (i.e., the geometrical construction).
Depending on the transistor configuration and materials used (e.g., silicon oxide, silicon nitride, carbon layers, metal oxides), the base-collector capacitance can have different structures and spread. The most significant contribution to the capacitance is due to the dielectrics used. In general the total base-collector capacitance has three components. More particularly, the total base-collector capacitance of the bipolar transistor is the sum of the capacitance in the active base silicon region and the parasitic capacitances occurring between the base connection areas and collector due to the dielectric between them.
This total base-collector capacitance can be written:CBC,total=CBC,active+CBC,dielectric,1+CBC,dielectric,2 
Therefore, the total base-collector capacitance (CCB,total) comprises an active capacitive component (CBC,active) due to the doping profile in the active device (between the emitter and the active collector region), a first dielectric component (CBC,dielectric,1) due to the capacitance of the dielectric layer comprised between the base connection region and the collector region, and a second dielectric component (CBC,dielectric,2) also due to the capacitance of the dielectric layer comprised between the base connection region and the collector region.